Re: [PATCH 1/2] gcc_qsort: source code changes

[Richard Biener <richard.guenther@gmail.com>]

On Thu, May 10, 2018 at 5:56 PM, Alexander Monakov <amonakov@ispras.ru> wrote:
>         * sort.cc: New file.
>         * system.h [!CHECKING_P] (qsort): Redirect to gcc_qsort.
>         * vec.c (qsort_chk): Use gcc_qsort.

Looks good to me.  As additional enhancement we might want to provide
(even unconditionally?)
the glibc qsort_r() interface.  I remember adding various globals to
pass down state to the comparator...

I agree self-tests might be good to have.  Also it looks like the
qsort-checking may now be somehow
embedded within our qsort implementation?

But all these things can be done as followup I think.

Thanks,
Richard.

> ---
>  gcc/sort.cc  | 232 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  gcc/system.h |   7 +-
>  gcc/vec.c    |   2 +-
>  3 files changed, 238 insertions(+), 3 deletions(-)
>  create mode 100644 gcc/sort.cc
>
> diff --git a/gcc/sort.cc b/gcc/sort.cc
> new file mode 100644
> index 00000000000..4faf6d45dc6
> --- /dev/null
> +++ b/gcc/sort.cc
> @@ -0,0 +1,232 @@
> +/* Platform-independent deterministic sort function.
> +   Copyright (C) 2018 Free Software Foundation, Inc.
> +   Contributed by Alexander Monakov.
> +
> +This file is part of GCC.
> +
> +GCC is free software; you can redistribute it and/or modify it
> +under the terms of the GNU General Public License as published by the
> +Free Software Foundation; either version 3, or (at your option) any
> +later version.
> +
> +GCC is distributed in the hope that it will be useful, but WITHOUT
> +ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> +FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
> +for more details.
> +
> +You should have received a copy of the GNU General Public License
> +along with GCC; see the file COPYING3.  If not see
> +<http://www.gnu.org/licenses/>.  */
> +
> +/* This implements a sort function suitable for GCC use cases:
> +   - signature-compatible to C qsort, but relaxed contract:
> +     - may apply the comparator to elements in a temporary buffer
> +     - may abort on allocation failure
> +   - deterministic (but not necessarily stable)
> +   - fast, especially for common cases (0-5 elements of size 8 or 4)
> +
> +   The implementation uses a network sort for up to 5 elements and
> +   a merge sort on top of that.  Neither stage has branches depending on
> +   comparator result, trading extra arithmetic for branch mispredictions.  */
> +
> +#ifdef GENERATOR_FILE
> +#include "bconfig.h"
> +#else
> +#include "config.h"
> +#endif
> +
> +#include "system.h"
> +
> +#define likely(cond) __builtin_expect ((cond), 1)
> +
> +#ifdef __GNUC__
> +#define noinline __attribute__ ((__noinline__))
> +#else
> +#define noinline
> +#endif
> +
> +/* C-style qsort comparator function type.  */
> +typedef int cmp_fn (const void *, const void *);
> +
> +/* Structure holding read-mostly (read-only in netsort) context. */
> +struct sort_ctx
> +{
> +  cmp_fn *cmp; // pointer to comparator
> +  char   *out; // output buffer
> +  size_t n;    // number of elements
> +  size_t size; // element size
> +};
> +
> +/* Helper for netsort. Permute, possibly in-place, 2 or 3 elements,
> +   placing E0 to C->OUT, E1 to C->OUT + C->SIZE, and so on. */
> +static void
> +reorder23 (sort_ctx *c, char *e0, char *e1, char *e2)
> +{
> +#define REORDER_23(SIZE, STRIDE, OFFSET)        \
> +do {                                            \
> +  size_t t0, t1;                                \
> +  memcpy (&t0, e0 + OFFSET, SIZE);              \
> +  memcpy (&t1, e1 + OFFSET, SIZE);              \
> +  char *out = c->out + OFFSET;                  \
> +  if (likely (c->n == 3))                       \
> +    memcpy (out + 2*STRIDE, e2 + OFFSET, SIZE); \
> +  memcpy (out, &t0, SIZE); out += STRIDE;       \
> +  memcpy (out, &t1, SIZE);                      \
> +} while (0)
> +
> +  if (sizeof (size_t) == 8 && likely (c->size == 8))
> +    REORDER_23 (8, 8, 0);
> +  else if (likely (c->size == 4))
> +    REORDER_23 (4, 4, 0);
> +  else
> +    {
> +      size_t offset = 0, step = sizeof (size_t);
> +      for (; offset + step <= c->size; offset += step)
> +       REORDER_23 (step, c->size, offset);
> +      for (; offset < c->size; offset++)
> +       REORDER_23 (1, c->size, offset);
> +    }
> +}
> +
> +/* Like reorder23, but permute 4 or 5 elements. */
> +static void
> +reorder45 (sort_ctx *c, char *e0, char *e1, char *e2, char *e3, char *e4)
> +{
> +#define REORDER_45(SIZE, STRIDE, OFFSET)        \
> +do {                                            \
> +  size_t t0, t1, t2, t3;                        \
> +  memcpy (&t0, e0 + OFFSET, SIZE);              \
> +  memcpy (&t1, e1 + OFFSET, SIZE);              \
> +  memcpy (&t2, e2 + OFFSET, SIZE);              \
> +  memcpy (&t3, e3 + OFFSET, SIZE);              \
> +  char *out = c->out + OFFSET;                  \
> +  if (likely (c->n == 5))                       \
> +    memcpy (out + 4*STRIDE, e4 + OFFSET, SIZE); \
> +  memcpy (out, &t0, SIZE); out += STRIDE;       \
> +  memcpy (out, &t1, SIZE); out += STRIDE;       \
> +  memcpy (out, &t2, SIZE); out += STRIDE;       \
> +  memcpy (out, &t3, SIZE);                      \
> +} while (0)
> +
> +  if (sizeof (size_t) == 8 && likely (c->size == 8))
> +    REORDER_45 (8, 8, 0);
> +  else if (likely(c->size == 4))
> +    REORDER_45 (4, 4, 0);
> +  else
> +    {
> +      size_t offset = 0, step = sizeof (size_t);
> +      for (; offset + step <= c->size; offset += step)
> +       REORDER_45 (step, c->size, offset);
> +      for (; offset < c->size; offset++)
> +       REORDER_45 (1, c->size, offset);
> +    }
> +}
> +
> +/* Helper for netsort. Invoke comparator CMP on E0 and E1.
> +   Return E0^E1 if E0 compares less than E1, zero otherwise.
> +   This is noinline to avoid code growth and confine invocation
> +   to a single call site, assisting indirect branch prediction. */
> +noinline static intptr_t
> +cmp1 (char *e0, char *e1, cmp_fn *cmp)
> +{
> +  intptr_t x = (intptr_t)e0 ^ (intptr_t)e1;
> +  return x & (cmp (e0, e1) >> 31);
> +}
> +
> +/* Execute network sort on 2 to 5 elements from IN, placing them into C->OUT.
> +   IN may be equal to C->OUT, in which case elements are sorted in place.  */
> +static void
> +netsort (char *in, sort_ctx *c)
> +{
> +#define CMP(e0, e1)                   \
> +do {                                  \
> +  intptr_t x = cmp1 (e1, e0, c->cmp); \
> +  e0 = (char *)((intptr_t)e0 ^ x);    \
> +  e1 = (char *)((intptr_t)e1 ^ x);    \
> +} while (0)
> +
> +  char *e0 = in, *e1 = e0 + c->size, *e2 = e1 + c->size;
> +  CMP (e0, e1);
> +  if (likely (c->n == 3))
> +    {
> +      CMP (e1, e2);
> +      CMP (e0, e1);
> +    }
> +  if (c->n <= 3)
> +    return reorder23 (c, e0, e1, e2);
> +  char *e3 = e2 + c->size, *e4 = e3 + c->size;
> +  if (likely (c->n == 5))
> +    {
> +      CMP (e3, e4);
> +      CMP (e2, e4);
> +    }
> +  CMP (e2, e3);
> +  if (likely (c->n == 5))
> +    {
> +      CMP (e0, e3);
> +      CMP (e1, e4);
> +    }
> +  CMP (e0, e2);
> +  CMP (e1, e3);
> +  CMP (e1, e2);
> +  reorder45 (c, e0, e1, e2, e3, e4);
> +}
> +
> +/* Execute merge sort on N elements from IN, placing them into OUT,
> +   using TMP as temporary storage if IN is equal to OUT.
> +   This is a stable sort if netsort is used only for 2 or 3 elements. */
> +static void
> +mergesort (char *in, sort_ctx *c, size_t n, char *out, char *tmp)
> +{
> +  if (likely (n <= 5))
> +    {
> +      c->out = out;
> +      c->n = n;
> +      return netsort (in, c);
> +    }
> +  size_t nl = n / 2, nr = n - nl, sz = nl * c->size;
> +  char *mid = in + sz, *r = out + sz, *l = in == out ? tmp : in;
> +  /* Sort the right half, outputting to right half of OUT. */
> +  mergesort (mid, c, nr, r, tmp);
> +  /* Sort the left half, leaving left half of OUT free.  */
> +  mergesort (in, c, nl, l, mid);
> +  /* Merge sorted halves given by L, R to [OUT, END). */
> +#define MERGE_ELTSIZE(SIZE)                     \
> +do {                                            \
> +  intptr_t mr = c->cmp (r, l) >> 31;            \
> +  intptr_t lr = (intptr_t)l ^ (intptr_t)r;      \
> +  lr = (intptr_t)l ^ (lr & mr);                 \
> +  out = (char *)memcpy (out, (char *)lr, SIZE); \
> +  out += SIZE;                                  \
> +  r += mr & SIZE;                               \
> +  if (r == out) return;                         \
> +  l += ~mr & SIZE;                              \
> +} while (r != end)
> +
> +  if (likely (c->cmp(r, l + (r - out) - c->size) < 0))
> +    {
> +      char *end = out + n * c->size;
> +      if (sizeof (size_t) == 8 && likely (c->size == 8))
> +       MERGE_ELTSIZE (8);
> +      else if (likely (c->size == 4))
> +       MERGE_ELTSIZE (4);
> +      else
> +       MERGE_ELTSIZE (c->size);
> +    }
> +  memcpy (out, l, r - out);
> +}
> +
> +void
> +gcc_qsort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
> +{
> +  if (n < 2)
> +    return;
> +  char *base = (char *)vbase;
> +  sort_ctx c = {cmp, base, n, size};
> +  long long scratch[32];
> +  size_t bufsz = (n / 2) * size;
> +  void *buf = bufsz <= sizeof scratch ? scratch : xmalloc (bufsz);
> +  mergesort (base, &c, n, base, (char *)buf);
> +  if (buf != scratch)
> +    free (buf);
> +}
> diff --git a/gcc/system.h b/gcc/system.h
> index 4abc321c71d..88dffccb8ab 100644
> --- a/gcc/system.h
> +++ b/gcc/system.h
> @@ -1202,11 +1202,14 @@ helper_const_non_const_cast (const char *p)
>  /* qsort comparator consistency checking: except in release-checking compilers,
>     redirect 4-argument qsort calls to qsort_chk; keep 1-argument invocations
>     corresponding to vec::qsort (cmp): they use C qsort internally anyway.  */
> -#if CHECKING_P
> +void qsort_chk (void *, size_t, size_t, int (*)(const void *, const void *));
> +void gcc_qsort (void *, size_t, size_t, int (*)(const void *, const void *));
>  #define PP_5th(a1, a2, a3, a4, a5, ...) a5
>  #undef qsort
> +#if CHECKING_P
>  #define qsort(...) PP_5th (__VA_ARGS__, qsort_chk, 3, 2, qsort, 0) (__VA_ARGS__)
> -void qsort_chk (void *, size_t, size_t, int (*)(const void *, const void *));
> +#else
> +#define qsort(...) PP_5th (__VA_ARGS__, gcc_qsort, 3, 2, qsort, 0) (__VA_ARGS__)
>  #endif
>
>  #endif /* ! GCC_SYSTEM_H */
> diff --git a/gcc/vec.c b/gcc/vec.c
> index 11924a80a2d..2941715a34a 100644
> --- a/gcc/vec.c
> +++ b/gcc/vec.c
> @@ -215,7 +215,7 @@ void
>  qsort_chk (void *base, size_t n, size_t size,
>            int (*cmp)(const void *, const void *))
>  {
> -  (qsort) (base, n, size, cmp);
> +  gcc_qsort (base, n, size, cmp);
>  #if 0
>  #define LIM(n) (n)
>  #else
> --
> 2.13.3
>